Enantioselective catalysis using chiral metal complexes provides one of the most general and flexible methods for achieving asymmetric organic reactions. Metallic elements possess a variety of catalytic activities, and permutations of organic ligands or other auxiliary groups directing the steric course of the reaction are practically unlimited. Efficient ligands must be endowed with, for example, suitable functionality, appropriate chirality, a structure capable of differentiating space either electronically or sterically and skeletal rigidity or flexibility.
Among the asymmetric organic reactions catalyzed by chiral transition metal complexes, asymmetric hydrogenation has been one of the best studied, due in large part to the fact that it is the basis for the first commercialized catalytic asymmetric process. See, for example, ApSimon, et al., Tetrahedron, 1986, 42, 5157.
Some of the more interesting of the asymmetric hydrogenation catalysts are those derived from BINAP [2,2'-bis(diphenylphosphino)-1,1'-binaphthyl]. See, for example, U.S. Pat. Nos.: 4,691,037; 4,739,084; 4,739,085; 4,764,629; 4,994,607; and 4,766,227. Unlike the more classical models of chira (asymmetric) molecules, chirality in the case of the BINAP-type compounds arises from the restricted rotation about the single bond joining the naphthalene rings. Isomers arising from this type of asymmetry are termed atropisomers.
BINAP-based Ru(II) and Rh(I) complexes induce high enantioselectivity in catalytic reactions. See Noyori and Takaya, Acc. Chem. Res., 1990, 23, 345.
The BINAP ruthenium complexes are dramatically different than the rhodium ones. They have been used to catalyze a variety of asymmetric hydrogenations, including the hydrogenation of enamides and alkyl and aryl-substituted acrylic acids. See Noyori, et al., Modern Synthetic Methods, 1989, 5, 115, incorporated herein by reference. Unlike the rhodium catalyzed reductions, ruthenium(II) carboxylate complexes possessing the BINAP ligand are efficient catalysts for the enantioselective hydrogenation of .alpha.,.beta.-unsaturated carboxylic acids. According to Ohta, et al, J. Org. Chem, 52, 3174 (1982), the carboxyl moiety of the substrate, and not other oxygen containing groups, is responsible for the stereoselective reaction. Asymmetric reductions of noncarboxyl-containing substrates by ruthenium complexes are inefficient.
The preparation of the BINAP-bearing ruthenium complexes, while not only sophisticated, is time consuming and expensive. Accordingly, it would be advantageous to be able to carry out these enantioselective transformations using more readily prepared catalysts.
In rhodium catalyzed asymmetric reactions, in situ methods of preparing the active catalysts are well established. However, such an in situ method to prepare ruthenium catalysts is not generally successful. See, for example, B. Heiser, et al., Tetrahedron Asymmetry, 2, 51 (1991). The necessity of synthesizing the catalyst in an extra step and complications due to catalyst instability are avoided by in situ catalyst generation.